NULLIF()

Description: NULLIF returns the value of the first argument, unless it is
equal to the second. In that case, NULL is returned.

Result type: Depends on input.

Syntax:

NULLIF (<exp1>, <exp2>)

Example:

select avg( nullif(Weight, -1) ) from FatPeople

This will return the average weight of the persons listed in FatPeople, excluding those
having a weight of -1, since AVG skips NULL data.
Presumably, -1 indicates “weight unknown” in this table. A plain
AVG(Weight) would include the -1 weights, thus skewing the result.

Note

In Firebird 1.0.x, where NULLIF is not available, you can
accomplish the same with the *nullif external
functions.